Traditional methods of rotor position estimation in a switched reluctance (SR) motor usually involve either: (1) injection of a signal into one of the phase windings during rotational movement, and identifying the resulting impedance using current sensors and/or flux sensors, or (2) bringing the rotor into alignment on a torque null position, then identifying impedance using the currents and fluxes inherent in the firing of the winding to determine impedance. These approaches have significant drawbacks. In this regard, a lookup table of impedance (inductance, core loss, resistance, capacitance, etc.) is used to find the starting position of the rotor. Moreover, movement may possibly be in a rotation direction opposite of that desired. Furthermore, a period of time must elapse for the position to stabilize before starting, depending upon the load friction and inertia, as well as any other connected load effects (e.g., damping).
One significant drawback to the foregoing methods is the need to use costly current sensors and/or flux sensors. Another drawback to these methods is that they are sensitive to the temperature of the motor and the applied line voltage.
It is recognized that tap voltages, as described in related U.S. patent application Ser. No. 09/257,867, now U.S. Pat. No. 6,150,778, and assigned to the assignee of the present application, may be utilized to determine rotor position. However, the system described in such patent application is best suited for determining rotor position after the rotor has begun spinning at a relatively high speed. It has been observed that the system is less suited for lower motor speeds, stopped or stalled motors, and during speed transients.
The present invention extends the concepts described in the prior patent application with regard to tap voltages signals, to provide an encoderless position detection method and apparatus which determines rotor position when the rotor is stationary.